Pulse width modulated transistor inverter circuits are well-known. State-of-the-art inverter circuits of this type generally operate with the pulses per cycle of the pulse width modulating waveform numbering 3 or 15. It is, of course, recognized that any number of pulses per cycle are possible. For a given output waveform and given number of pulse per cycle, the harmonic content may be calculated and the result compared with a sine wave as a measure of quality.
Numerous problems prevent inverter circuits of this type from getting a low harmonic content sine wave. Typical of these problems are those that arise from load and source characteristics as well as switching transients in the inverter. The presence of finite variable switching time in addition to the non-linearity of the switches and the load further contribute to the imperfections present in the inverter transistor circuit sine wave output.
The desirability of reducing the harmonic content of an inverter circuit output is recognized in the patent to Klein, U.S. Pat. No. 3,979,662, in which two inverters are employed. The two inverters are connected to supply electrical power to a common load. The harmonic content is reduced by interconnecting the inverters by means of transformers to cancel some of the harmonic voltages, and a main component of output power of one of the inverters is used directly without being transformed. The patent to Klein does not, as the invention to be described more fully hereinafter, provide for an inverter circuit having a reduced harmonic output signal accomplished by summing a filtered fundamental PWM signal with a controlled signal from another inverter, which controlled signal is the function of the sum of a desired waveform signal and the filtered fundamental PWM signal.
The problem of harmonic content present in an inverter circuit output signal is also addressed in the patent to Taddec et al., U.S. Pat. No. 4,067,057. The Taddec et al. patent teaches switching the transistors in the inverter circuit at a rate significantly higher than desired output frequencey of the AC output power. The increased switching frequency, however, results in increased switching losses becoming a problem in the transistors. The invention to be described hereinafter avoids this problem by employing in a main inverter circuit that has a switching rate of a fixed frequency.